The present invention relates to semiconductor processing and, more particularly, to a system and method which provide for efficient plasma production in a region directly above a semiconductor wafer. A major objective of the present invention is to provide for confinement of energetic electrons near the wafer with increased symmetry to enhance plasma and reactive natural species production and, thereby, enhance uniformity and rate of etching and deposition across a wafer.
Much of recent technological progress is identified with increasingly miniature integrated circuits which have been providing greater functional density at lower costs. Semiconductor processing technology has provided for increased functionality, in part, by permitting smaller features to be patterned onto a semiconductor wafer, and has provided for lower costs, in part, by developing equipment which can handle larger wafers, thereby increasing the number of circuits that can be made together. The 1980s have witnessed a rapid progression from 3-inch wafers with 3-micron features to 8-inch wafers with half micron features, and from 16-kilobit memories to one- and four-megabit memories, with 16- and 64-megabit memories in development.
Semiconductor processing typically involves a series of photolithographic procedures in which: 1) a layer of material is deposited or grown on a wafer, 2) a layer of photoresist is deposited over the material; 3) the photoresist is exposed to radiation according to a predetermined pattern defined by a mask or stepper program; 4) developer is used to remove resist, with either the exposed or unexposed resist being left in place; 5) the wafer is etched so that exposed material is removed, while material beneath the resist is retained; and 6) the remaining resist is removed to expose the patterned layer of material.
The trends of smaller features and larger wafers combine to make deposition and etch uniformity both more important and more difficult to achieve. Attainment of small features requires that photolithographic procedures be applied to very thin layers. This means that depositions and etches must be precisely controlled. Some approaches which were effective with larger feature sizes are becoming less applicable for submicron feature sizes. For example, wet chemical etches tend to be isotropic so that they etch laterally into material which is supposed to be protected by photoresist, requiring feature tolerances to be imposed. Furthermore, the surface tension associated with wet chemical etches impairs their conformance to small openings needed to define small features at the wafer surfaces exposed below the photoresist.
Plasma-based reactions have become increasingly important, providing for precisely controlled thin-film depositions and etches. A plasma is a mixture of positive and negative ions and electrons which is typically characterized by a visible glow. A plasma can be formed by capacitively coupling electric energy at radio frequencies, via a metal cathode, into electrons which can then impact gas molecules, creating more ions and free electrons. The positive ions are pushed outward by mutual repulsion toward surfaces including the cathode used in generating the electric field. Thus, a wafer supported over the cathode will be bombarded by positive ions which can form a deposited layer or cause a material to be etched, depending on the composition of the plasma.
Since the cathode has a large negative potential in a thin sheath region above and adjacent to the wafer surface, ions are accelerated so that they impact the wafer with large velocity components perpendicular to the plane of the wafer. These ions then bombard areas of material exposed due to the absence of photoresist and provide a high reaction rate on horizontal wafer surfaces with minimal effect on vertical sidewall features. Thus, plasma reactions are particularly well suited for precise pattern definition of small features.
Electrons have a charge equal in magnitude to that of the positive ions while being thousands of times lighter. Without some form of confinement, the more energetic electrons would diffuse away from the wafer, depleting the plasma of electrons capable of sustaining it and, thus, rendering it less dense immediately above the wafer. This diminished density results in a slower etch or deposition. The effect of electron depletion is apparent in the sheath over the wafer surface from which electrons are repelled. This sheath lacks the glow of the nearby plasma and is commonly referred to as a "dark space".
Energetic electrons can be confined to the plasma using a magnetic field which is parallel to the wafer surface. Electrons can be considered as being pulled into orbit about magnetic field lines so as to prevent their escape in a direction perpendicular to the magnetic field. A magnetic field along a wafer surface can be established using a pair of electro-magnets disposed on opposite sides of the wafer and arranged coaxially and parallel to each other so that their mutual axis extends parallel to the wafer surface. The magnetic fields generated by the coils are aligned so that they add to produce a field which is generally parallel to the wafer surface and uniform along the mutual axis.
The magnetic field B confines energetic electrons by forcing them into helical orbits about magnetic field lines. An electron in orbit about a magnet field parallel to and above a wafer progresses along a spiral from West to up to East to down. An electric field accelerates electrons in a direction opposed to the electric field, so that the downward electric field above a wafer in a conventional reactor system accelerates electrons in an upward direction.
An electron at the Western extreme of its orbit is traveling upward so that its speed is increased by the electric field. While at the top extreme of its orbit, the electron has acquired its maximum speed and is traveling in an Eastward direction. While at the Eastern extreme of its orbit, an electron undergoes a decrease in speed since its motion is aligned with the electric field. Minimum speed is reached at the bottom extreme of the orbit when the electron is traveling Westward. Thus, an orbiting electron travels fast Eastwardly and slow Westwardly.
The net effect is an Eastward E.times.B drift of the electron. Cumulatively, the energetic electrons, and thus the plasma they sustain, are subject to this Eastwind drift. This drift results in a plasma which is shifted Eastward relative to the wafer rather than being centered over it. This shift is a potential source of non-uniform depositions and etches.
There are various ways of dealing with this plasma shift. Electrons can be allowed to drift around an electrode to form a closed loop which goes under the wafer and then returns to the plasma region over the wafer. This requires a reaction vessel design in which the electric and magnetic fields all around the wafer support permit the required closed loop motion.
A time-averaged cylindrical symmetry can be achieved by rotating the wafer within the fields. However, rotating the wafer induces undesirable mechanical motion in the plasma, which can increase contamination. This mechanical motion can be avoided by rotating the magnetic field rather than the wafer, which can be done using two pairs of coils and phasing the currents though the coils so that the vector sum of the magnetic fields rotates in the plane parallel to the wafer surface. While excellent results have been achieved in such a system, the costs involved with the additional coils and current modulations make it less desirable. In addition, optimal uniformity on the largest wafers is more difficult to attain with coils of finite size in this configuration.
What is needed is a system and method for confining electrons to the desired region which avoids the problems associated with plasma shift. Preferably, plasma shift should be avoided even on an instantaneous basis so that time-averaging need not be relied on.